This invention relates to the recording of information, on rotating discs, in a form which can be written and read optically through the projection of a focused laser beam. The information is typically recorded in tracks on the surface of the disc. With the disc rotating, in order to determine the precise starting point (heading) in the desired track at which writing or reading should begin, it is necessary to provide a datum point or position signal common to all tracks.
A conventional method of providing this datum point is shown in FIG. 1, which is a schematic perspective view showing an optical disc 1 being driven by a disc drive. The rotating disc 2 of the disc drive receives its rotational energy from the motor 5 through the center shaft 3. Under the rotating disc 2, the timing disc 4 is coaxially supported with the rotating disc 2 on the center shaft. On the surface of the timing disc 4 is a timing mark used as the datum. A light source 6 projects light onto the surface of the timing disc 4; and a detector 7 detects the amount of this light which is reflected from the surface. In order properly to position the optical disc 1 with respect to the timing disc 4 and to prevent relative rotational movement between the two (since movement would cause reading or recording errors), a projection 2' is provided on the edge of rotating disc 2, and a recess 1' is provided in the edge of the optical disc 1. Before rotation begins, when the optical disc 1 is first placed on the rotating disc 2, it is done so as to couple the projection 2' with the recess 1'. Determination of the location of information stored on the optical disc 1 is made possible by projecting the light on the surface of the timing disc 4 and detecting (through the detector 7) the variations in the amount of light reflected by the mark.
In the case of this kind of disc drive, however, as it is necessary to have a timing disc 4 in addition to the rotating disc 2, the disc drive is that much more complicated. Furthermore, when installing an optical disc 1 on the rotating disc 2, the operator must be careful to match the recess 1' with the projection 2'. Also, production of optical discs is made more complicated by the need to provide recesses 1' in the edges.